1. Field of the Invention
The present invention pertains to seismic data waveform processing and more particularly to the estimation of amplitude spectra using up-going and down-going wavefields.
2. Related Prior Art
Much of seismic exploration today is being done offshore, that is, in the coastal waters io within several hundred miles of land. In some cases, offshore means slightly off the coast, such as wells off the coast of Louisiana or California. In other cases, offshore can be two hundred miles off the coast, such as the Outer Banks oil fields off the coast of Newfoundland. However, both cases have a common problem, seismic data acquisition is complicated by the presence of both upgoing and downgoing acoustic waves.
In situations where both upgoing and downgoing are present, pressure and particle velocity detectors may be used to separate upward traveling waves, (U), from downward traveling waves, (D). Pressure and particle velocity detectors on the water bottom can be used to separate upward traveling wavefields, (U), from polarity reversed downward traveling wavefields, (D), as demonstrated in U.S. Pat. No. 5,754,492 issued to Starr.
Many ways of determining the up going and down going vector wavefields may be used in separate upward traveling wavefields, from polarity reversed downward traveling wavefields. One method includes locating pairs of seismic receivers at the surface of a water column. This surface or reflecting interface may be the air water interface at the top surface or the water bottom when water column reverberations in marine seismic data is considered. This surface may also be the interface between geologic layers in subsurface formations. Further, in some cases, seismic data may be collected with pressure and particle velocity response receivers located at the same location while in other cases, seismic data may be collected with vertically spaced receivers. In the one case, up going and down going wavefields are directly detected by the two receivers in the one location. In the other case, up going and down going wavefields can be separated by comparing the sequential outputs of the receivers.
For the specific case where the pressure and particle velocity detectors are located on the water bottom, the impulse response of the earth recorded on the U data is the same as that which is recorded on the D data except for a linear phase shift associated with the two-way travel time through the water column, (Z). An autocorrelation is the time domain equivalent of the amplitude spectra in the frequency domain. In cases where a single measurement is made of a signal, such as single phone recordings, the amplitude spectra is estimated by autocorrelating the signal. Autocorrelation of a signal causes problems for a signal containing noise. The noise correlates with itself giving an error in the spectral estimation.